A data network comprises a number of source nodes, each source node receiving traffic from numerous traffic sources, and a number of sink nodes, each sink node delivering data to numerous traffic sinks. The source nodes can be connected to the sink nodes directly or through core nodes. Source nodes and sink nodes are often paired to form edge nodes, where a source node and sink node of an edge node share memory and control.
Each link between two nodes may comprise multiple channels. An optical multi-channel link uses Wavelength Division Multiplexing (WDM). WDM allows a given optical link to be divided into multiple channels, where a distinct stream of data may be transmitted on each channel and a different wavelength of light is used as a carrier wave to form each of the multiple channels within the optical link.
The performance, efficiency, and scalability of a telecommunications network depend heavily on the nodal degree and the directly related network diameter. The degree of a specific node is a measure of the number of nodes to which the specific node directly connects. The term topological reach is used herein to refer to the number of sink nodes that a source node can reach directly or through the network core. The diameter of a network is a measure of the maximum number of hops along the shortest path between any two nodes. For a given network capacity, the higher the nodal degree, the smaller the network diameter becomes, and a small network diameter generally yields high performance and high efficiency. On the other hand, for a given nodal degree, scalability generally increases with the network diameter, but to the detriment of network efficiency. It is therefore advantageous to increase the nodal degree to the highest limit that technology permits.
In a network based on channel switching, a source node connects to destination sink nodes through channels, each channel being associated with a wavelength. The topological reach of a source node, i.e., the number of destination sink nodes that the source node can reach without switching at an intermediate edge node, is then limited by the number of channels emanating from the source node, which is typically significantly smaller than the number of edge nodes in the network. Time-sharing enables fine switching granularity and, hence, a high topological reach. Effective time-sharing in a bufferless-core network requires that the edge nodes be time-locked to the core nodes, that all nodes be fast-switching, and that a path between two edge nodes traverses a single optical core node. A node X is said to be time-locked to a node Y if, at any instant of time, the reading of a time-counter at node X equals the sum of a reading of an identical time-counter at node Y and the propagation time from node X to node Y, where the time counters at nodes X and Y have the same period, and the propagation delay is measured relative to said period. Thus, if each of several edge nodes transmits a pulse, when its time-counter reading is σ, to a specific core node, the pulses from the edge nodes arrive at the core node when the time-counter reading of the core node is also σ.
TDM (time-division-multiplexing) and burst switching are two modes of network time sharing. In TDM, data is organized in a time-slotted frame of a predefined duration and a path from a source node to a sink node may be allocated one or more time slots. In burst switching, data packets are aggregated into bursts, generally of different sizes, and the bursts are switched in the core towards destination sink nodes, where each burst is disassembled into constituent packets. Both TDM and burst switching can be exploited to increase the nodal degree, hence reduce the network diameter. The application of TDM in an optical-core network is described in Applicant's U.S. patent application Ser. No. 09/960,959, filed on Sep. 25, 2001 and titled “Switched channel-band Network,” which is incorporated herein by reference.
Prior-art burst switching has attractive features but has two main drawbacks: burst-transfer latency and burst loss. In a closed-loop scheme, a source node sends a request to a core node for transferring a burst, the request including a destination and size of the burst, and waits for a message from the core node, where the message acknowledges that the optical switch in the core node is properly configured, before sending the burst. In an open-loop scheme, the burst follows the burst transfer request after a predetermined time period, presumably sufficient to schedule the burst transfer across the core, and it is expected that, when the burst arrives at the core node, the optical switch will have been properly configured by a controller of a core node. It is noted that even if a very long time gap is kept between a burst-transfer request and the data burst itself, the lack of buffers at the core node may result in burst loss and a significant idle time.
In the closed-loop scheme, the time delay involved in sending a burst transfer request and receiving an acceptance before sending a burst may be unacceptably high, leading to idle waiting periods and low network utilization in addition to requiring large storage at the edge nodes.
In the open-loop scheme, a burst may arrive at a core node before the optical switch can be configured to switch the burst and the burst may be lost. Furthermore, the fact that the burst has been lost at the core node remains unknown to the source node for some time and a lost burst would have to be sent again after a predefined interval of time.
In a wide-coverage network, the round-trip propagation delay from an edge node, comprising a paired source node and a sink node, to a core node can be of the order of tens of milliseconds. This renders closed-loop burst scheduling inappropriate. In closed-loop switching, a source node and a core node must exchange messages to determine the transmission time of each burst. The high round-trip delay requires that the source node have a sizeable buffer storage. On the other hand, open-loop burst scheduling, which overcomes the delay problem, can result in substantial burst loss due to unresolved contention at the core nodes. It is desirable that data bursts formation at the source nodes and subsequent transfer to respective optical core nodes be performed with low delay, and that burst transfer across the core be strictly loss-free. It is also desirable that the processing effort and transport overhead be negligibly small.
A burst scheduling method and a mechanism for burst transfer in a composite-star network is described in the applicant's U.S. patent application Ser. No. 09/750,071, filed on Dec. 29, 2000, and titled “Burst Switching in a High-Capacity Network”, the contents of which are incorporated herein by reference. According to the method, a burst-transfer request is sent to a controller of a core node after a burst has been formed at a source node. High efficiency is, however, maintained by burst scheduling and burst-transfer pipelining. The burst transfer across the optical-core is loss-free. However, a burst has to wait at its source node for a period of time slightly exceeding a round-trip delay between the source node and a selected core node. In a network of global coverage, the burst-transfer latency may exceed a high value, 20 milliseconds for example, for a significant proportion of the traffic.